you aren't foolish, but those are a terrible deal
20g H tank from Petco 39.99
Bulkheads (5x @ $9 (shipped) $45
overflow $50
hole drilling (5x @ $15) $75
silicone $3
total to do it yourself = $213 vs $290
thats $77 bucks, you can pretty much build a stand and canopy for free or else an entire retro kit for lighting
convenience factor is something to consider, however in this hobby most of the pleasure and satisfaction comes from DIY projects, and like I mentioned, the $77 bucks you save would give you a number of things "for free"...retro kit,
closed loop pump, canopy/stand materials, etc etc etc, hell you could even do the unthinkable and buy a coral from a pet store for full price